This patent application claims priority based on Japanese patent application, 2000-1998 filed on Jan. 7th, 2000.
1. Field of Invention
This invention relates to an electric potential detector, a device tester, and a method of detecting electric potential.
2. Description of Related Art
A device tester has to accurately measure a voltage output from a device to secure a reliability of the testing. Moreover, an electric current that flows into the device and the electric current that flows out of the device also have to be measured accurately.
FIG. 1 shows a conventional electric potential detector. The electric potential V that is input from an input terminal 30 is input to both an A/D convertor 810 and analog comparators 820 and 830. Each of the A/D convertor 810 and the comparators 820 and 830 are comprised for performing a determination of the digital value of the electric potential V and a judgement (pass/fail judgement) of whether the electric potential V exists within an allowable range. The pass/fail judgement can be known by comparing the maximum and minimum value of the allowable range and the value of the electric potential V.
Details of the pass/fail judgement will be explained. A maximum value of an allowable range of an electric potential is provided to the plus side of an input terminal of the comparator 820 from the D/A convertor 840, and a comparison is performed between the maximum value and an analog value of the electric potential V, which is input to a minus side of an input terminal. When the analog value of the electric potential V is larger than the maximum value, the comparator 820 sets the logic of the HFAIL signal as L level and informs that the electric potential V is xe2x80x9cHIGH FAILxe2x80x9d. On the other hand, when the analog value of the electric potential V is smaller than the maximum value, the comparator 820 outputs a signal of H level as an HFAIL signal and informs that the electric potential V is xe2x80x9cHIGH PASSxe2x80x9d.
Similarly, a minimum value is input to a minus side of an input terminal of the comparator 830 from a D/A convertor 850, and the electric potential V is input to a plus side of an input terminal. The comparator 830 sets the logic of the LFAIL signal to be L level (LOW FAIL) or H level (LOW PASS) according to the comparison result.
In the configuration of FIG. 1, the determination of the digital value (A/D conversion) is performed at the A/D convertor 810, and the pass/fail judgement is performed at the comparator 820 and 830, independently. It cannot be avoided that a few differences arise among the characteristics of the A/D convertor 810 and the comparator 820 and 830 during the manufacturing process. Therefore, there is a problem that inconsistency of the result arises such that the judgement is a fail although the digital value is within the allowable range. To solve the above mentioned problems, an electric potential detector shown in FIG. 2 was used.
FIG. 2 is a circuit diagram that shows another configuration of the conventional electric potential detector. First, the analog electric potential V input from the input terminal 30 is converted to digital by the A/D convertor 810. The digital value of the electric potential V output from the A/D convertor 810 is then input to both of the digital comparator 920 and 930. Each of the maximum value and the minimum value are further input to the comparator 920 and 930, respectively, from the limit resistor 940 and 950, and the pass/fail judgement is performed as in the same way as in the configuration of FIG. 1.
As is clear from the above explanation, the pass/fail judgement is performed for the output of the A/D convertor 810 in the configuration of FIG. 2. Thus, the inconsistency of the result as in the case of FIG. 1 does not arise. However, the electric potential V has to be A/D converted even when it is sufficient to perform only the pass/fail judgement so that it takes a long time for a judgement.
Therefore, it is an object of the present invention to provide an electric potential detector, device tester, and a method of detecting electric potential that has a configuration of performing a judgement at high speed without causing an inconsistency of testing results in view of the above issues. This object is achieved by combinations of characteristics described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
To solve the above problems, the first aspect of the present embodiment is characterized in that the first embodiment comprises a comparator that compares an input electric potential, which is an object to be detected, with a referential value and outputs a comparison result; and a control circuit that has: a judgement means for performing a judgement of pass/fail of the input electric potential based on the comparison result obtained by providing an allowable limit value, which is a value indicating an allowable limit of an input electric potential, as the referential value; and a value determination means for finding a value of the input electric potential based on the comparison result obtained by providing the referential value to the comparator by varying the referential value.
The second aspect of the present invention is characterized in that the control circuit provides the comparator with a maximum value and a minimum value of an allowable range of the input electric potential, which is the allowable limit value, as the referential value and judges that the input electric potential is a failure when the input electric potential is judged as higher than the maximum value or judged as lower than the minimum value by the comparator.
The third aspect of the present invention is characterized in that the value determination means detects a transition point of the comparison result by selecting a value from a plurality of candidate values one after another arbitrarily as the referential value and determines a value of the input electric potential based on a candidate value that corresponds to the transition point.
The fourth aspect of the present invention is characterized in that the comparator includes a first and a second comparator to which the maximum value and the minimum value are input respectively; and the control circuit judges whether the input electric potential is allowable at a high potential side based on a comparison result of the first comparator and judges whether the input electric potential is allowable at a low potential side based on a comparison result of the second comparator.
The fifth aspect of the present invention characterized in that the input electric potential is analog; and the judging means provides the maximum value and the minimum value to the comparator at different timings; and the electric potential detector further comprises: a retaining means that retains the input electric potential while the maximum value and the minimum value are provided to the comparator.
The sixth aspect of the present invention is characterized in that the electric potential detector further comprises: an input electric potential retaining means for retaining the input electric potential during selection of a value from the plurality of candidate values one after another arbitrarily as the referential value.
The seventh aspect of the present invention is a device tester that comprises: a test pattern generator that generates an input test pattern, which is to be input to a device under test (DUT), and an expectation pattern, which is to be output from the DUT into which the input test pattern is input; a device testing comparator for judging a propriety of the DUT according to whether an output pattern, which is output from the DUT according to the input test pattern that is input to the DUT, and the expectation pattern matches; a power supply for supplying electric power to the DUT; and a terminal potential detecting circuit for detecting an electric potential of a terminal potential of the DUT; characterized in that: the terminal potential detecting circuit has: a sample potential comparator for outputting a comparison result by comparing a sample electric potential, which is input for observing the electric potential of the terminal, with a referential electric potential; a judging means for judging a pass/fail of the sample electric potential based on the comparison result obtained by providing an allowable limit potential, which is an electric potential that indicates an allowable limit of the sample electric potential, as the referential electric potential; and a control unit that includes a value determination means for finding a value of the sample electric potential based on the comparison result obtained by providing the referential electric potential to the sample potential comparator by varying the referential electric potential.
The eighth aspect of the present invention characterized in that the method comprises: a judging step for judging whether an input electric potential, which is an object to be tested, is within an allowable range and judging as xe2x80x9cfailurexe2x80x9d when an input electric potential is not within an allowable range; and a retrieving step for retrieving a detailed value of the input electric potential corresponding to a judgement, which is given as the xe2x80x9cfailurexe2x80x9d in the judging step.
The ninth aspect of the present invention is characterized in that the judging step judges whether the input electric potential is within the allowable range by comparing a maximum and minimum of the allowable range and the input electric potential.
The tenth aspect of the present invention is characterized in that the retrieving step is the step that detects a transition point of a comparison result by comparing a value, which is selected from a plurality of candidate values as a reference, with the input electric potential one after another and determines the detailed value according to a candidate value corresponding to the transition point.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.